1. (Field of the Invention)
This invention relates to an air-fuel ratio control system for controlling the amount of fuel containing alcohol supplied to an internal combustion engine.
2. (Prior Art)
Gasoline, which is conventionally used as a fuel for internal combustion engines, has a stoichiometric air-fuel ratio of 14.7, and it is desirable to control the amount of fuel injected into an engine so that the air-fuel ratio of a mixture of the fuel and air becomes equal to the stoichiometric air-fuel ratio when the engine is in a normal operating condition. When the engine is in a heavily-loaded condition, however, the temperature of combustion chambers of the engine rises so high that knocking is liable to occur. Therefore, conventionally, when it is detected that the engine is in a predetermined high load region, the fuel injection amount is increased according to load on the engine so that the combustion chambers are cooled due to an increased amount of latent heat required for evaporation of the increased amount of fuel and the air-fuel ratio is richer than the stoichiometric one to thereby prevent compression ignition and hence knocking from occurring.
On the other hand, alcohol has been recently used as a substitute fuel for gasoline. However, methyl alcohol, for example, has a stoichiometric air-fuel ratio of 6.4, which is largely different from the value of gasoline. Therefore, when methyl alcohol is used as a substitute fuel, it is required to control the fuel injection amount such that it is larger than when gasoline is used as fuel. Further, methyl alcohol is not only used as fuel containing 100% alcohol, but also used in a mixed fuel containing 85% alcohol and the balance of gasoline, which is known as M85. Accordingly, it is required to change the fuel injection amount depending on the concentration of alcohol in the fuel. Under these circumstances, the present assignee has already been proposed, by Japanese Patent Application No. 2-336479, an air-fuel ratio control system which detects the concentration of alcohol in the fuel, and changes the fuel injection amount depending on the detected alcohol concentration.
Alcohol has a larger value of latent heat of vaporization than gasoline, and a higher octane number, which makes knocking less liable to occur even when the engine is under a heavily-loaded condition. Therefore, it is less required to increase the fuel injection amount under a heavily-loaded condition of the engine. However, due to the above-mentioned small air-fuel ratio of alcohol, it is required to inject the substitute fuel containing alcohol in a larger amount than gasoline. For these reasons, if the fuel injection amount is increased when the load on the engine is in the predetermined high region as described with respect to the case in which gasoline is used as fuel, the injection amount of alcohol becomes excessive, which can bring about the disadvantage that the excessive amount of alcohol spoils lubrication between the piston and the cylinder, and between the connecting rod and the piston pin.